Ever wonder what these are?

[SARtech]

Strange looking nests.
I did. Looked it up when I got home. "Tent Caterpillars"

[Rivman]

Very ugly looking things - they can strip the foliage off trees in no time !
We usually don't get them till mid/late summer.

[CaraRose]

I always thought those were gypsy moths? *going to Wikipedia...*

[barb_48]

We are having the same problems. When I do find them in my trees I cut the branch off, and through them out.

[SmartWombat]

Many caterpillars are gregarious in the early instars, and spin a web.
Commonest around here are the small tortoiseshell Aglais urticae.

Tent caterpillar is a generic term, not a species, there are 6 species in the USA.

http://books.google.co.uk/books?id=g...esult&resnum=8


Your picture doesn't look like Gypsy moth, those have front 5 segments with blue spots, and rear 6 segments with red spots, and no lateral line.

https://www.allposters.co.uk/-sp/A-G..._i3990477_.htm


Your caterpillars look close to the Eastern tent caterpillar Malacosoma americanum the Forest tent caterpillar doesn't have the solid dorsal line. But I don't have a complete set of images of all the tent caterpillar species and sub-species.